I tend to think, that Roman victories over Hellenic armies had less to do with phalanx and legion and more to do with general military prowess. It is true, that since Zama, Roman legions beat up all phalanxes. But that is because since Zama until break up of Roman Empire, Roman legions beat up anything. With just few notable exceptions.

I have read that the Macedonians lost so consistently against the Romans because, after Alexander, generals forgot how to use the phalanx in concert with cavalry correctly. They got into pushing fights with their phalanxes and sent their cavalry to fight the enemy cavalry, instead of the combined arms tactics of using the phalanx to hold the enemy in place and flanking them with the cavalry.

I have read that the Macedonians lost so consistently against the Romans because, after Alexander, generals forgot how to use the phalanx in concert with cavalry correctly. They got into pushing fights with their phalanxes and sent their cavalry to fight the enemy cavalry, instead of the combined arms tactics of using the phalanx to hold the enemy in place and flanking them with the cavalry.

How true is that?

I think it is very problematic statement. First of all, Alexander and his father Philip were military geniuses. That their successors did not manage to live up to their standard do not have to be result of them forgetting something. Hellenistic empires were in near permanent state of wars so they certainly did not lacked general skill. Personally I do not see Hellenistic kings and generals not using cavalry or light infantry along their phalanxes. They might not have handled them as good as Alexander but they did not neglected them.

I do not attribute Roman victories to Hellenistic military been defective. I attribute it to Roman quality. Romans simply were very good at war at that point in time. They after all just defeated Hannibal -man who was if not equal than second only to Alexander himself (after taking bloody lessons from him for decades). And for next 500 years, Romans basically steamrolled anything which stood in their way militarily.

I have read that the Macedonians lost so consistently against the Romans because, after Alexander, generals forgot how to use the phalanx in concert with cavalry correctly. They got into pushing fights with their phalanxes and sent their cavalry to fight the enemy cavalry, instead of the combined arms tactics of using the phalanx to hold the enemy in place and flanking them with the cavalry.

How true is that?

Take Pydna for example, phalanx were clearly defeating the legions. But as legions back off, the Macedonians just pushed forward with phalanx. And as a result, the inherent weakness of phalanx surfaced, phalanx lost its coherence, and legions beat them. It's clear cavalry played no role here, and phalanx was used offensively instead of defensively to hold enemy line as Alexander did.

This is wrong. The most common helmets during Republican age were the Attic and the Italo-Corinthian ones.

Muscled armour that used officials were of Greek origen.

Roman swords before adoption of Gladius were xyphos and kopis.

Earlier roman armies, if we consider Etruscan armies, most probably fought with aspis.

Those weren't copies, they were leftovers from the age when Rome was pretty much a fringe Hellenic city state, or just products of cultural osmosis in the Mediterranean. There was no copying going on at the time of the legions, not from the Greeks, and pitifully little Hellenistic gear remained in use by the time of the great civil wars - mostly limited to largely decorative helmets and cuirass designs worn by aristocratic officers. The average grunt was by then armed much more like a Celt - an oblong shield, javelins, a sword, a shirt of mail and a Celtic-style helmet.

As for Pydna, the phalanx wasn't beating the Romans, it was pushing them back. The withdrawal worked perfectly for the Romans, even though I doubt anyone anticipated the phalanx breaking up this way.

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phalanx was used offensively instead of defensively to hold enemy line as Alexander did.

Any large body of men will eventually break up, so you better use the time you have. Either way, Alexander was hardly ever on the defensive, but he benefited from more flexible organization.

Here is the stele of one of Caracalla's Spartans. Marcus Aurelius Alexys served between 214-217, dying at age 40, 'having campaigned against the Persians'. Due to the Edict of Caracalla, a great majority of male citizens of Sparta took the Emperor's praenomen and nomen, as thanks for their Roman citizenship. Hence the name, Marcus Aurelius.

The legions strength over the phalanx was its extreme efiiciency.A legion going head on against a phalanx would lose because thats the strength of the phalanx,in Cynoscephalae the phalanxes did much harm to the legion, and at Asculum [not sure if it was Asculum] the phalanx demolished the legion.The legion however could get organized into formation much faster,was suitable to much more varieties of terrain and to many more situations than a phalanx,but head-on there is no way that a legion would beat a phalanx. If anyone has any questions about what the pila would do to a phalanx,I read that the pila did little to no damage to the phalanx dont have the time to provide sources but feel free to search on your own.Overall as I said in the beginning of my post what made the phalanx obsolete was the extreme advantadge of efficiency in favor of the legion.